I'm trying to force keyframes in mp4 files every second at 25fps

Using : ffmpeg -i in.mp4 -force_key_frames "expr:gte(t,n_forced*1)" out.mp4
I get 1 keyframe every 10 seconds every time.
I've tried the other methods in the docs with the same result
Using either libx264 or NVENC produces the same result.
Sourcefile input makes no difference.
Checking in virtualdubMod shows keyframes every 250 frames

Obviously I'm missing something - can anyone explain or tell me how to do this.
thanks